Additive compositions that are capable of changing the viscosity of various liquids are of considerable interest for a number of commercial and industrial applications. The viscosity of hydrocarbon solvents such as benzene, hexane, heptane, and cyclopentane can be altered by the addition of various thickening agents, to suit a particular end use application. Moreover, the use of additives for altering the viscosity of supercritical carbon dioxide (CO2) has also become an important objective.
Supercritical CO2 (sometimes referred to herein as simply “CO2”) is of great interest as a solvent in chemical processing, because it is non-flammable, relatively non-toxic, and naturally abundant. These properties have prompted the use of CO2 as an organic solvent for polymerization; along with many other applications, such as a solvent in biocatalysis; and as a raw material in chemical synthesis.
Another important application for supercritical CO2 relates to oil recovery from underground formations. In enhanced oil recovery (EOR), a flooding agent is pumped into oil-bearing formations, to move the petroleum to exit wells. Water is a typical flooding agent, but its use has various limitations. For example, water is not a good solvent for oil; and intimate contact between petroleum and water results in cross-contamination that requires the remediation of large volumes of organic-contaminated water.
Supercritical CO2 is a better solvent for oil than water; and would be a more environmentally-sustainable flooding agent than water. However, the viscosity of supercritical CO2 is too low to effectively recover petroleum from the formation. Rather than sweeping the oil before it, CO2 has the tendency to finger its way through the petroleum, bypassing most of the oil. The recovery of oil therefore entails the injection of very large amounts of purchased and recycled CO2, over extended periods of time.
Various techniques have been developed to try to accommodate the low viscosity of supercritical CO2 in an EOR application. One injection of water and supercritical CO2, i.e., the “WAG” process. This example that has been shown to be beneficial on a limited basis involves the formation of CO2 emulsions or foams which decrease the solvent's mobility. Another example is based on the alternate technique can reduce the CO2 saturation, thereby decreasing the CO2's relative permeability, and increasing its ability to sweep through more of the formation.
While the WAG process is generally recognized by most operators as superior to the continuous injection of CO2, and can make recovery more economical, it still results in most of the oil being left behind in the formation. Moreover, the process introduces operational difficulties, such as the need to produce, separate, process, and re-inject large volumes of water. It also increases the time required to inject the entire CO2 “slug”. This in turn can undesirably delay the completion of the overall oil recovery project.
Active research has involved designing additives to raise the viscosity of supercritical CO2, to render the solvent more practical. However, the various additives have often not been entirely satisfactory, for various reasons. As an example, high-molecular weight organic polymers such as those based on copolymers of styrene and fluorinated acrylates do have the ability to thicken the supercritical CO2, but can sometimes be very expensive; and can also be toxic, e.g., if the fluoroacrylate contains a strand of eight fluorinated carbons.
With these concerns in mind, new materials that can beneficially alter the viscosity of hydrocarbons and specialty solvents like supercritical CO2 would be welcome in the art. The materials should be relatively benign to the environment; and economical to make and use. The materials should also be very compatible with both EOR processes, as well as other oil and petroleum extraction techniques, such as hydraulic fracturing.